The present invention relates to a method for manufacturing molded articles from a ceramic composite structure, in particular from a tri-silicon tetranitride metal silicide composite structure.
Ceramic materials in their potential with regard to higher operational temperatures are significantly superior to comparable materials, for example metal alloys. In this context, tri-silicon tetranitride materials are particularly suitable for various applications, in particular those involving high temperatures, these materials distinguishing themselves in their superior mechanical as well as electrical properties, when in corresponding composite structures in combination with electrically conductive compounds. Non-oxidic ceramic materials having a silicon base have a high resistance to thermomechanical stress, and also, in high temperature ranges up to, for example, 1300xc2x0 C., are substantially resistant to oxidation and corrosion. Another further important aspect of non-oxidic ceramic materials lies in the controlled adjustment of their electrical properties, particularly in material combinations and composite structures.
German Patent No. 37 34 274 describes ceramic materials having a base of silicon nitride, aluminum nitride, and xcex2-sialon in combination with secondary phases of various silicides, carbides, borides, and nitrides of transitional metal elements. In accordance with the secondary-phase content, these materials possess adjustable electrical properties. The adjustable specific values for the electrical resistance of these materials is between 1*1013 and 1*10xe2x88x924 xcexa9cm at room temperature, and they demonstrate a positive dependence on temperature (PTC effect). The strength level of these composite materials, produced in this manner, is not below 200 Mpa. The method employed there for manufacturing composite materials that are resistant to great heat is designated as single-axis hot-pressing, which has particular disadvantages with regard to the molding of articles manufactured from these composite materials, can have anisotropic material properties due to the direction of pressing, and is applicable only as a charging method, i.e., not as a continuous method. In addition, this method requires high temperatures and pressures.
Furthermore, it is known to manufacture electrically insulating composite materials that are resistant to high temperatures having a base of tri-silicon tetranitride having metal silicides of the formula Msi2 and M5Si3, M being a transitional metal or a main group metal, manufacturing taking place by gas pressure sintering at a pressure of 100 bar N2 (See German Patent Application No. 195 00 832, and European Patent Application No. 0 721 925). As a result of the necessary high pressures of up to 400 Mpa, among other reasons, gas pressure sintering is expensive and cumbersome. Hot-pressing also requires axial pressures of up to 30 Mpa, it is expensive, and it is only applicable for simple components.
A method according to the present invention, in contrast to the conventional methods described above, has the advantage that greater free space for shaping is possible for molded articles that are thermomechanically highly stressed, in combination with the controlled variation of their electrical properties. In addition, complicated geometric structures can be realized by an essentially more favorable processing in the green state. This is achieved in that, first, in a multistep pressing process, the article to be molded is pressed in an isostatically cold state and then is rendered in the desired form. As a result, there is no need for a cumbersome hard-processing, for example after the hot-sintering, such as is required in a single-axis pressing process.
In another embodiment, after the cold isostatic molding press step, a first sintering at atmospheric pressure using an inert gas is carried out. Thus the shaping undertaken is further solidified (strengthened).
In an advantageous manner, the final sintering is carried out in a protective gas partial pressure, preferably nitrogen, from 2 to 10 bar. In this context, the sintering temperature of this sintering is between 17000xc2x0 and 1900xc2x0 C. From a phase diagram of the components employed, A and B, it can be inferred under these conditions that only the pure phases A and B are present, and possible mixed phases do not occur. Thus it is particularly avoided that, for example, non-conductive phases or phase transitions or poorly conductive phases lessen or decisively impair the desired electrical properties of the sintered final product.
In another embodiment of the present invention, the sintering takes place in a range of the protective gas partial pressure log p(N2), which is defined by an upper limit Y1 and a lower limit Y2. In this context, the nitrogen partial pressures Y1 and Y2 and the sintering temperature T have the following relation to each other:
Y1=7.1566xc2x7ln(T)xe2x88x9252.719
Y2=9.8279xc2x7ln(T)xe2x88x9273.988
As a result, it is assured that in the phase diagram of the system of the chosen materials, only the pure phases also occur in this area. As a result of the variable relation between temperature and pressure, an optimal range is thus defined which, by changing the temperature and pressure, permits maintaining the optimal method parameters in a further range, without, in this context, resulting, for example, in thermal decomposition, particularly of a thermally less stable component such as a nitride. In addition, as a result of this defined relation between pressure and temperature, mixed phases having undesirable profile characteristics are avoided in a simple way.
Preferably, in the ceramic composite structure, tri-silicon tetranitride is used as component A and a metal silicide as component B. As silicide, the most commonly used metal silicides, for example, MoSi2, can be considered. Tri-silicon tetranitride, in contrast to its homologues of boron and nitrogen, has greater hardness and a better sintering capacity.
It is advantageous to use as protective gas either nitrogen or a mixture of nitrogen and an inert gas, for example argon, so that a potential decomposition reaction of the nitride employed, in particular, Si3N4, after a subsequent equilibrium reaction, is substantially suppressed: 
The application of the Le Chatelier Principle, by increasing the concentration of one component of the equilibrium, thus makes it possible in a simple manner to increase the thermodynamic stability of Si3N4. Thus the sintering temperatures themselves lying above the decomposition point of Si3N4 are usable for the sintering process. In addition, it is thus possible to reduce the concentration of sintering additives in Si3N4, which are often disruptive of the electrical properties, for example, aluminum oxide or yttrium oxide, to a value of under 10% by weight. It is similarly advantageous that the total pressure can be controlled and adjusted in a simple manner by adding a second inert gas, e.g., argon. This has a particularly advantageous influence on the sintering product with regard to the achieved material thickness of the two sintering variants, without altering the electrical properties.